linux-kernel - Re: PATCH 1/3] blktrace: support per-partition tracing

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Date:	Tue, 14 Apr 2009 16:53:41 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Tom Zanussi <tzanussi@...il.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"Theodore Ts'o" <tytso@....edu>, Shawn Du <duyuyang@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-btrace@...r.kernel.org
Subject: Re: PATCH 1/3] blktrace: support per-partition tracing

>>> Lets note that this approach still has the (long existing) 
>>> limitation that only one device can be block-traced at a time.
>>>
>> No, both userspace blktrace and ftrace-plugin trace can trace
>> more than one device at a time.
>>
>>  # btrace /dev/sda /dev/dm-0
>>
>> or
>>
>>  # echo 1 > /sys/block/sda/trace/enable
>>  # echo 1 > /sys/block/dm-0/trace/enable
> 
> When they are independent - but not multiple partitions at a time:
> 

The only way to trace multipl partitions is to trace the whole
sda, or set trace/start_lsa and trace/end_lsa properly.

>  [root@...ebaran ~]# echo 1 > /sys/block/sda/sda1/trace/enable 
>  [root@...ebaran ~]# echo 1 > /sys/block/sda/sda2/trace/enable 
>  -bash: echo: write error: Device or resource busy

That's because struct blk_trace is attached to struct request_queue,
sda1 and sda2 share the same rq:

  # echo 1 > /sys/block/sda/sda1/trace/enable 
  # cat /sys/block/sda/sda1/trace/enable 
  1
  # cat /sys/block/sda/sda2/trace/enable 
  1

>  [root@...ebaran ~]#
> 
> Nor did i see any "trace all block IO in the system" kind of 
> functionality in blktrace. (or maybe there's one that i missed?)
> 

Right, and the only way to do this is 'blktrace sda sdb sdc ...'
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